1. Field of the Invention
This invention pertains generally to a disk drive array system used as an external storage unit for a data processing system and, more particularly, to a system for monitoring and controlling synchronization of the rotation of a plurality of disks in a disk drive array system.
2. Prior Art
FIG. 1 shows a conventional disk drive array system having three magnetic disk drives 1a-1c. The magnetic disk drives 1a-1c are each provided with a common external synchronizing signal 2 from a controller 3. The common external synchronizing signal 2 is generated using a suitable oscillator 21 and is used to ensure that each of the disk drives 1a-1c are operating in synchronization with its counterparts. Synchronization of the disk drives 1a-1c enables parallel data transfer and improved performance. The external synchronizing signal 2 is produced once for each synchronous revolution of the disk drives 1a-1c.
Each of the magnetic disk drives 1a-1c includes a magnetic disk 4a, 4b and 4c, for recording information magnetically, a magnetic head 5a, 5b and 5c, for recording and reproducing information from its magnetic disk, a spindle 22a, 22b and 22c on which its magnetic disk rests, a spindle motor 6a, 6b and 6c for rotating the corresponding spindle and magnetic disk, and an index sensor 7a, 7b and 7c for detecting the rotation of the corresponding spindle motor 6a, 6b and 6c. Each disk drive 1a, 1b and 1c also includes a phase locked loop (PLL) 8a, 8b and 8c, for maintaining the phase of the spindle motor 6a, 6b and 6c to keep the motor synchronized with the external synchronizing signal 2. A suitable PLL is described in U.S. Pat. No. 4,870,843 "Parallel Drive Array Storage System". Each magnetic disk drive 1a, 1b and 1c further includes a period detecting circuit 9a, 9b and 9c, for determining whether the rotational period of the corresponding magnetic disk (i.e., the time it takes the disk to complete one rotation) is within a predetermined range on the basis of outputs from the associated index sensor 7a, 7b and 7c.
FIG. 3a shows a more detailed circuit diagram of the period detecting circuit 9a. The other period detector circuits 9b and 9c may be of like construction. The period detector circuit 9a includes an inverter 41 that receives the input signal "IN". The inverter 41 may be implemented using a conventional integrated circuit (IC), like the Texas Instruments 74LS04 chip. The inverted output of the inverter 41 is passed to a re-triggerable single shot multi-vibrator 45. The input signal "IN" is also passed to another retriggerable single shot multi-vibrator 43. The multi-vibrators 43 and 45 may be implemented using the Texas Instruments 74LS123 chip. The non-complementary outputs of the multi-vibrators 43 and 45 are passed to as the D inputs to respective D-type flip-flops 47 and 49. The input signal is tied to the T inputs of the flip-flops 47 and 49. These D-type flip-flops may be implemented using Texas Instruments 74L874 chips.
The non-complementary output from D-type flip-flop 47 is passed to the input of a NOR gate 51. The other input to the NOR gate 51 is the complementary output from the D-type flip-flop 49. This NOR gate 51 may be implemented using the Texas Instruments 74LS00 IC. The output from the NOR gate 51 is complemented by another inverter 53 to generate the output for the period detector circuit 9a.
When the period of the "IN" signal is within the predetermined range of the time T.sub.0, the output of the period detector 9a is logically high. However, when the period is greater or less than the time T.sub.0, the output of the period detector 9a is logically low. The time period T.sub.1 for the first multi-vibrator 43 is equal to 1.1 T.sub.0, whereas the time period T.sub.2 for the second multi-vibrator 44 is equal to 0.9 T.sub.0.
Lastly, each magnetic disk drive 1a, 1b and 1c includes a read-write (R/W) circuit 10 (10a, 10b), for controlling the read and write operations that are performed from/to the associated magnetic disk.
When the rotational periods of the three magnetic disk drives 1a-1c are within a desirable range, "logically high" ready (RDY) signals 11a-11c are provided to an AND circuit 12 in the controller 3 to signal that the rotational period is stable.
Since the conventional disk array system is constructed as described above, the rotation of the respective magnetic disks 4a, 4b and 4c is disturbed by any fluctuations in the external synchronizing signal. More particularly, if the line, over which the external synchronizing signal 2 is carried, becomes opened or shorted or, alternatively, if the period of the external synchronizing signal 2 becomes unstable, the reference signals produced by the phase controllers 8a, 8b and 8c, for controlling the rotation of the disks also fluctuate so that the rotation of the magnetic disks 4a, 4b and 4c is disturbed. Other factors such as noise may contribute to fluctuations in the external synchronizing signal.
Writing and reading operations are adversely affected by the fluctuation in the rotation of the magnetic disks 4a, 4b and 4c so that information is not suitably stored and reproduced to/from the magnetic disks. Further, the position of the magnetic heads 5a, 5b and 5c fluctuate in extraordinary up or down movements during the rotation of the magnetic disks 4a, 4b and 4c, and the magnetic heads may contact the magnetic disks, thereby causing serious damage, such as a head crush.